amd/olivehillplus/romstage.c: remove not useful variable 'halt'

The variable 'halt' is not useful and results in a compile error
because of:
   1b2f2a07 Introduce halt()
build error:
src/mainboard/amd/olivehillplus/romstage.c: In function 'cache_as_ram_main':
src/mainboard/amd/olivehillplus/romstage.c:43:15: error: declaration of 'halt' shadows a global declaration [-Werror=shadow]
In file included from src/include/cpu/x86/lapic.h:6:0,
                 from src/mainboard/amd/olivehillplus/romstage.c:29:
src/include/halt.h:31:32: error: shadowed declaration is here [-Werror=shadow]
cc1: all warnings being treated as errors
make: *** [build/mainboard/amd/olivehillplus/romstage.pre.inc] Error 1

Change-Id: Id67a0dcb192fb6478115e489f46bfb07021afd90
Signed-off-by: WANG Siyuan <SiYuan.Wang@amd.com>
Signed-off-by: WANG Siyuan <wangsiyuanbuaa@gmail.com>
Reviewed-on: http://review.coreboot.org/7847
Reviewed-by: Edward O'Callaghan <eocallaghan@alterapraxis.com>
Reviewed-by: Patrick Georgi <pgeorgi@google.com>
Reviewed-by: Bruce Griffith <Bruce.Griffith@se-eng.com>
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>
Tested-by: build bot (Jenkins)